Liquid crystal display device

ABSTRACT

A liquid crystal display device includes a first electrode to which a signal is supplied through a first switching element and a second electrode to which a signal is supplied through a second switching element in each pixel region on a substrate, wherein liquid crystal is driven in response to a potential difference between the first electrode and the second electrode. In such a constitution, the first electrode is formed as one electrode of a first holding capacitance which is constituted by sandwiching an insulation film between the first electrode and a signal line and, at the same time, the second electrode is formed as one electrode of a second holding capacitance which is constituted by sandwiching an insulation film between the second electrode and a signal line.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional application of U.S. application Ser.No. 10/893,437 filed Jul. 19, 2004. Priority is claimed based on U.S.application Ser. No. 10/893,437 filed Jul. 19, 2004, which claims thepriority of Japanese Patent Application No. 2003-209552 filed Aug. 29,2003, all of which is incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display device.

2. Description of the Related Art

There has been known a liquid crystal display device in which a pair ofsubstrates are arranged to face each other in an opposed manner withliquid crystal therebetween, each one of pixel regions formed on aliquid-crystal-side of one substrate includes a pixel electrode and acounter electrode thereon and the optical transmissivity of the liquidcrystal is controlled by generating an electric field between the pixelelectrode and the counter electrode.

With respect to such a liquid crystal display device, there has beenknown a liquid crystal display device in which the above-mentionedrespective electrodes are arranged by way of an insulation film, whereinone electrode is formed on the whole region of a center portion in theinside of the pixel region, while another electrode is formed as a groupof electrodes which are overlapped to one electrode and have a so-calledcomb-teeth shape. Further, these electrodes are formed of a lighttransmitting conductive layer.

Such a liquid crystal display device is disclosed in JP-A-2002-90781 (orcorresponding U.S. Pat. No. 6,562,645), for example.

Further, in JP-A-2000-338462, there has been disclosed a liquid crystaldisplay device having the constitution in which a first video signaltransmitted from the first drain signal line is supplied to oneelectrode through the first thin film transistor which is turned on inresponse to a scanning signal transmitted from a gate signal line, and asecond video signal transmitted from the second drain signal line issupplied to another electrode through the second thin film transistorwhich is turned on in response to the above-mentioned scanning signal.

In the liquid crystal display device having such a constitution, twodrain signal lines are arranged with respect to two thin filmtransistors formed in one pixel, wherein one voltage from one drainsignal line is applied to one thin film transistor and another voltagefrom another drain signal line is applied to another thin filmtransistor. Here, one voltage assumes a positive pole and anothervoltage assumes a negative pole and these voltages constitute voltagesin one frame period.

Due to such an operation, the differential voltage between the voltageof one electrode and the voltage of another electrode inside the pixelis applied to the liquid crystal. Further, to enable the AC driving ofthe liquid crystal voltage, the polarities of one electrode and anotherelectrode are exchanged in the next frame period.

Due to such a constitution, compared to an in-plane switching typeliquid crystal display device which arranges a thin film transistor inone pixel and in which the potential of a pixel electrode which isconnected to the thin film transistor is, in a state that a potential ofanother electrode is fixed, subjected to AC driving with respect to thefixed voltage, the AC differential voltage can be halved.

In view of the above, it is possible to decrease the driving voltage ofthe liquid crystal and hence, the liquid crystal display device of lowpower consumption can be obtained.

SUMMARY OF THE INVENTION

However, even with the provision of such a constitution, when a screenof the liquid crystal display device becomes large-sized, the powerconsumption is increased. Further, when the pixel electrode is used asone electrode and the gate signal line or the capacitance signal line isused as another electrode, the liquid crystal display device assumes amode in which only one polarity is charged depending on a displaypattern and hence, the wiring delay of the gate signal line or thecapacitance signal line is increased. Accordingly, it has been pointedout that when a square window pattern is displayed with respect to thebackground, a strip-like shadow which is referred to as a so-calledcrosstalk occurs in the direction of the gate signal line or thecapacitance signal line.

On the other hand, in the liquid crystal display device described inJP-A-2000-338462 which forms two thin film transistors in one pixel, acapacitance element having the structure which is formed by stacking oneelectrode and another electrode with an insulation film therebetween isprovided. The liquid crystal display device having such a structurerequires no capacitance signal line and hence, there is no possibilitythat the above-mentioned crosstalk is generated. However, thecapacitance element is configured such that output voltages of tworespective thin film transistors are merely connected to theabove-mentioned respective electrodes. Accordingly, during the holdingperiod in which a gate-OFF voltage is applied to the thin filmtransistor, the potential of the capacitance element assumes a floatingstate. In view of the above, it has been found that there may arise adrawback that the voltage value is not fixed and is changedcorresponding to the parasitic capacitance between the potential of thecapacitance element and the potential of the gate signal line or thelike.

Particularly, it has become apparent based on an experiment which theinventors carried out that when the gate potential of the thin filmtransistor is changed from an ON state to an OFF state, there may arisea drawback that an operational point is worsened, that is, the electrodepotential is remarkably lowered due to the parasitic capacitancegenerated between the gate electrode which is constituted of a portionof the gate signal line and a source electrode.

Assuming that the output voltages from the above-mentioned two thin filmtransistors are transmitted to the holding capacitance electrodes whichare independent from each other and these holding capacitance electrodesare overlapped to the capacitance signal lines with an insulation filmtherebetween thus forming the capacitance element, the lowering ofnumerical aperture of the pixels is unavoidable.

The present invention has been made under such circumstances and anadvantage of the present invention is to provide a liquid crystaldisplay device which can eliminate the possibility of a drawback that anoperational point is worsened due to the lowering of a potential of apixel electrode when a gate potential of a thin film transistor ischanged from an ON state to an OFF state.

To briefly explain the summary of representative inventions among theinventions disclosed in this specification, they are as follows.

(1)

In a liquid crystal display device according to the present invention,for example, which includes a first electrode to which a signal issupplied through a first switching element and a second electrode towhich a signal is supplied through a second switching element in eachpixel region on a substrate, wherein liquid crystal is driven inresponse to a potential difference between the first electrode and thesecond electrode,

the improvement is characterized in that the first electrode is formedas one electrode of a first holding capacitance which is constituted bysandwiching an insulation film between the first electrode and a signalline and, the second electrode is formed as one electrode of a secondholding capacitance which is constituted by sandwiching an insulationfilm between the second electrode and a signal line.

(2)

The liquid crystal display device according to the present invention is,for example, on the premise of the constitution (1), characterized inthat the first electrode and the second electrode are formed of a lighttransmitting conductive film, the first electrode and the secondelectrode are formed as different layers by way of an insulation film,one electrode is formed on a most portion of the pixel region, andanother electrode is formed of a group of electrodes which areoverlapped to one electrode.

(3)

The liquid crystal display device according to the present invention is,for example, on the premise of the constitution (1) or (2),characterized in that a signal line which constitutes another electrodeof the first holding capacitance is a first drain signal line, and asignal line which constitutes another electrode of the second holdingcapacitance is a second drain signal line.

(4)

The liquid crystal display device according to the present invention is,for example, on the premise of the constitution (1) or (2),characterized in that a signal line which constitutes another electrodeof the first holding capacitance and a signal line which constitutesanother electrode of the second holding capacitance are formed of acapacitance signal line.

(5)

The liquid crystal display device according to the present invention is,for example, on the premise of the constitution (4), characterized inthat a capacitance value of the first holding capacitance issubstantially equal to a capacitance value of the second holdingcapacitance.

(6)

The liquid crystal display device according to the present invention is,for example, on the premise of the constitution (5), characterized inthat the first electrode and the second electrode are formed of a lighttransmitting conductive film, the first electrode and the secondelectrode are formed on different layers by way of an insulation film,one electrode is formed on a most portion of the pixel region, andanother electrode is formed of a group of electrodes which areoverlapped to one electrode.

(7)

In a liquid crystal display device according to the present invention,for example, which includes a first electrode to which a signal issupplied from a first drain signal line through a first switchingelement and a second electrode to which a signal is supplied from asecond drain signal line through a second switching element in eachpixel region on a substrate, wherein liquid crystal is driven inresponse to a potential difference between the first electrode and thesecond electrode,

the improvement is characterized in that the first electrode and thesecond electrode are formed on different layers by way of an insulationfilm with respect to the first drain signal line and the second drainsignal line, and

portions of the first electrode and the second electrode are overlappedto the first drain signal line and the second drain signal line.

(8)

The liquid crystal display device according to the present invention is,for example, on the premise of the constitution (7), characterized inthat the first electrode has portions thereof overlapped to the firstdrain signal line and the second drain signal line respectively, and thesecond electrode has portions thereof overlapped to the first drainsignal line and the second drain signal line respectively.

(9)

The liquid crystal display device according to the present invention is,for example, on the premise of the constitution (8), characterized inthat the portions of the first electrode which are overlapped to thefirst drain signal line and the portions of the second electrode whichare overlapped to the first drain signal line are substantially at thesame positions, and the portions of the first electrode which areoverlapped to the second drain signal line and the portions of thesecond electrode which are overlapped to the second drain signal lineare substantially at the same positions.

(10)

The liquid crystal display device according to the present invention is,for example, on the premise of the constitution (8) or (9),characterized in that the portions of the first electrode which areoverlapped to the first drain signal line, the portions of the secondelectrode which are overlapped to the first drain signal line, theportions of the first electrode which are overlapped to the second drainsignal line, and the portions of the second electrode which areoverlapped to the second drain signal line respectively constituteholding capacitances.

(11)

The liquid crystal display device according to the present invention is,for example, on the premise of the constitution (8) or (9),characterized in that values of respective holding capacitances formedby the portions of the first electrode which are overlapped to the firstdrain signal line, the portions of the second electrode which areoverlapped to the first drain signal line, the portions of the firstelectrode which are overlapped to the second drain signal line, and theportions of the second electrode which are overlapped to the seconddrain signal line fall all together within a scope of 50% to 200%.

(12)

The liquid crystal display device according to the present invention is,for example, on the premise of the constitution (11), characterized inthat the capacitance value of the holding capacitance of the portions ofthe first electrode which are overlapped to the first drain signal lineand the capacitance value of the holding capacitance of the portions ofthe first electrode which are overlapped to the second drain signal lineare substantially equal and, at the same time, the capacitance value ofthe holding capacitance of the portions of the second electrode whichare overlapped to the first drain signal line and the capacitance valueof the holding capacitance of the portions of the second electrode whichare overlapped to the second drain signal line are substantially equal.

(13)

The liquid crystal display device according to the present invention is,for example, on the premise of the constitution (8) or (9),characterized in that the portions of the first electrode which areoverlapped to the first drain signal line and the portions of the secondelectrode which are overlapped to the first drain signal line as well asthe portions of the first electrode which are overlapped to the seconddrain signal line and the portions of the second electrode which areoverlapped to the second drain signal line are arranged in a staggeredmanner.

(14)

In a liquid crystal display device according to the present invention,for example, which includes a first electrode to which a signal issupplied from a first drain signal line through a first switchingelement and a second electrode to which a signal is supplied from asecond drain signal line through a second switching element in eachpixel region on a substrate, wherein liquid crystal is driven inresponse to a potential difference between the first electrode and thesecond electrode,

the improvement is characterized in that the liquid crystal displaydevice includes holding capacitance signal lines,

the holding capacitance signal lines are overlapped to a pluralityportions of the first electrodes by way of a first insulation film, and

the holding capacitance signal lines are overlapped to the secondelectrodes at regions between a plurality portions of the firstelectrodes by way of a first insulation film and a second insulationfilm.

(15)

The liquid crystal display device according to the present invention is,for example, on the premise of the constitution (14), characterized inthat with respect to overlapped areas where the holding capacitancesignal line is overlapped with the first electrode and the secondelectrode, the overlapped area where the holding capacitance signal lineis overlapped with the second electrode is larger than the overlappedarea where the holding capacitance signal line is overlapped with thefirst electrode.

(16)

The liquid crystal display device according to the present invention is,for example, on the premise of the constitution (14), characterized inthat the holding capacitance formed by the holding capacitance signalline and the first electrode and the holding capacitance formed by theholding capacitance signal line and the second electrode aresubstantially equal.

(17)

The liquid crystal display device according to the present invention is,for example, on the premise of any one of the constitutions (14), (15)and (16), characterized in that the first electrode and the secondelectrode are formed of a light transmitting conductive film, the firstelectrode and the second electrode are formed as different layers by wayof an insulation film, one electrode is formed on a most region of thepixel region, and another electrode is formed of a group of electrodeswhich are overlapped to one electrode.

Here, the present invention is not limited to the above-mentionedconstitutions and various modifications are conceivable withoutdeparting from the technical concept of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view showing one embodiment of a pixel of a liquidcrystal display device according to the present invention and FIG. 1B isan equivalent circuit diagram corresponding to FIG. 1A;

FIG. 2A is a cross-sectional view taken along a line II(a)-II (a) inFIG. 1A and FIG. 2B is a cross-sectional view taken along a lineII(b)-II(b) in FIG. 1A;

FIG. 3 is a plan view showing one embodiment of the liquid crystaldisplay device according to the present invention;

FIG. 4A and FIG. 4B are waveform charts and timing charts of signalssupplied to pixels of the liquid crystal display device according to thepresent invention

FIG. 5A is a plan view showing another embodiment of the pixel of theliquid crystal display device according to the present invention andFIG. 5B is an equivalent circuit diagram corresponding to FIG. 5A;

FIG. 6A is a cross-sectional view taken along a line VI(a)-VI (a) inFIG. 5A and FIG. 6B is a cross-sectional view taken along a lineVI(b)-VI(b) in FIG. 5A.

FIG. 7 is a cross-sectional view showing another embodiment of the pixelof the liquid crystal display device according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A liquid crystal display device according to the present invention isexplained hereinafter in conjunction with drawings.

Embodiment 1

FIG. 3 is a plan view showing one embodiment of a liquid crystal displaydevice according to the present invention. On a liquid-crystal-sidesurface of one substrate GLS1 out of respective substrates which faceeach other in an opposed manner with liquid crystal therebetween, gatesignal lines GL which extend in the x direction and are arranged in they direction in parallel are formed. Further, on the liquid-crystal-sidesurface of one substrate GLS1, drain signal lines DL which extend in they direction and are arranged in the x direction in parallel are formed.

These respective drain signal lines DL are repeatedly arranged such thatthe drain signal lines DL are arranged close to each other in theneighboring direction, the drain signal lines DL are arranged remotefrom each other in the neighboring direction, the drain signal lines DLare arranged close to each other in the neighboring direction, . . . Arectangular region which is surrounded by a pair of neighboring gatesignal lines GL and a pair of drain signal lines DL which are arrangedremote from each other (a portion surrounded by a bold line in thedrawing) constitutes a pixel region.

To each pixel region, a scanning signal is supplied from the gate signalline GL (for example, at a lower side in the drawing) which selects agroup of pixels arranged in the x direction in parallel including apixel in the pixel region. Further, a first video signal is supplied tothe pixel from the drain signal line DL at the left side in the drawing(the first drain signal line DLL), while a second video signal issupplied to the pixel from the drain signal line DL at the right side inthe drawing (the second drain signal line DLR). The detailedconstitution of the pixel region is described in detail later.

The above-mentioned respective gate signal lines GL have, for example,one ends thereof connected to a scanning signal drive circuit SCC and ascanning signal is sequentially supplied to the pixels by the scanningsignal drive circuit SCC. Further, the drain signal lines DL have, forexample, one ends thereof connected to a video signal drive circuit IMCand the video signals are supplied to the pixels by the video signaldrive circuit IMC at the timing of supplying of the scanning signal.

Here, the scanning signal drive circuit SCC and the video signal drivecircuit IMC are driven in response to a signal transmitted from acontroller CNTL and input signals such as a video signal and the likeare supplied to the controller CNTL from the outside.

FIG. 1A is a plan view showing one embodiment of the constitution of thepixel region and FIG. 1B is an equivalent circuit diagram depictedcorresponding to the constitution shown in FIG. 1A. FIG. 2A is across-sectional view taken along a line II(a)-II(a) in FIG. 1A and FIG.2B is a cross-sectional view taken along a line II(b)-II(b) in FIG. 1A.

On the surface of the transparent substrate GLS1, the gate signal linesGL which extend in the x direction and are arranged in the y directionin parallel are formed. These gate signal lines GL are formed so as tosurround.the pixel region together with the first drain signal line DLLand the second drain signal line DLR described later.

With respect to the pixel region, a first pixel electrode BPX is formedover the most region except for a slight peripheral region in thevicinity of lines, that is, over the whole region of a center portion.On portions of respective sides of the first pixel electrode BPX whichextend in the direction perpendicular to the gate signal lines GL,extension portions which extend parallel to the extending direction ofthe gate signal lines GL are formed, and these respective extensionportions are formed to be overlapped with the first drain signal lineDLL and the second drain signal line DLR described later.

The first pixel electrode BPX is, for example, formed of a lighttransmitting conductive layer and, as a material thereof, for example,ITO (Indium Tin Oxide), ITZO (Indium Tin Zinc Oxide), IZO (Indium ZincOxide), SnO₂ (Tin Oxide), In₂O₃ (Indium Oxide) or the like can beselected.

The extension portion of the first pixel electrode BPX formed at theleft side in the drawing is configured to be overlapped to the firstdrain signal line DLL thus forming an electrode Cbstl which constitutesthe holding capacitance.

Further, the extension portion of the first pixel electrode BPX formedat the right side in the drawing is configured to be overlapped to thesecond drain signal line DLR thus forming an electrode Cbstr whichconstitutes the holding capacitance.

On a surface of the transparent substrate GLS1 on which the gate signallines GL and the first pixel electrodes BPX are formed, a firstinsulation film GI is formed in a state that the first insulation filmGI also covers the gate signal lines GL and the first pixel electrodesBPX.

Further, above an upper surface of the gate signal line GL which isarranged in the inside of the pixel region, two thin film transistorsTFTL, TFTR which are spaced apart along the extending direction of thegate signal line GL are formed by way of the first insulation film GI.Here, one thin film transistor TFTL is referred to as the first thinfilm transistor and another thin film transistor TFTR is referred to asthe second thin film transistor.

Both of the first thin film transistor TFTL and the second thin filmtransistor TFTR have the same cross-sectional structure, while theirplaner structures are arranged to be approximately symmetrical. Further,by forming a semiconductor layer AS on the first insulation film GI andby forming a drain electrode and a source electrode on the semiconductorlayer AS, an MIS (Metal Insulator Semiconductor) transistor having theinverse staggered structure which adopts a portion of the gate signalline GL as the gate electrode and the first insulation film GI as a gateinsulation film is formed.

The drain electrodes and the source electrodes of the first thin filmtransistor TFTL and the second thin film transistor TFTR aresimultaneously formed along with the formation of the first drain signallines DLL and the second drain signal lines DLR.

The drain electrode of the first thin film transistor TFTL is formed byextending a portion of the drain signal line DLL to a surface of thesemiconductor layer AS, while a source electrode SD is formed in aspaced apart manner from the drain electrode by a length correspondingto a channel length of the first thin film transistor TFTL. The sourceelectrode SD is formed such that the source electrode SD slightlyextends into the inside of the pixel region to enable the connection ofthe source electrode SD with the first pixel electrode BPX. In the samemanner, the drain electrode of the second thin film transistor TFTR isformed by extending a portion of the second drain signal line DLR to thesurface of the semiconductor layer AS, while the source electrode SD isformed in a spaced apart manner from the drain electrode by a lengthcorresponding to a channel length of the thin film transistor TFTR. Thesource electrode SD is formed such that the source electrode SD slightlyextends into the inside of the pixel region for enabling the connectionof the source electrode SD with the second pixel electrode UPX.

A second insulation film PAS is formed on the surface of the transparentsubstrate GLS1 in a state that the second insulation film PAS alsocovers the first thin film transistors TFTL and the second thin filmtransistors TFTR. The second insulation film PAS has a function of aprotective film which prevents a direct contact of the above-mentionedrespective thin film transistors TFT with the liquid crystal.

Then, on a surface of the second insulation film PAS, the second pixelelectrodes UPX which are constituted of a group of a plurality ofstrip-like electrodes which extend in the y direction and are arrangedin parallel in the x direction in the drawing, for example, are formedin a state that the second pixel electrodes UPX are overlapped to thefirst electrodes BPX, wherein the second pixel electrodes UPX are formedof a light transmitting conductive layer. Here, the extending directionof the group of the plurality of strip-like electrodes may be the xdirection. Further, it may be possible to adopt the multi-domainconstitution by providing a plurality of extending directions and makingrespective extending directions different from both of the x directionand the direction perpendicular to the x direction.

Here, an end portion at the thin film transistor TFTR side of theelectrode which constitutes the second pixel electrode UPX has a portionthereof extended to a position above the source electrode SD of thesecond thin film transistor TFTR and is connected with the sourceelectrode via a through hole CNT2 which penetrates the second insulationfilm PAS which is disposed below the second thin film transistor TFTR.

In this case, the connection of the source electrode SD of theabove-mentioned first thin film transistor TFTL and the first pixelelectrode BPX is performed through a contact hole CNT1 formed in thesecond insulation film PAS and a contact hole CNT2 formed in the secondinsulation film PAS and the first insulation film GI in a penetratingmanner using a material layer SITO which is made of the same material asthe second pixel electrode UPX. Due to such a constitution, a step ofpenetrating the first insulation film GI and a step of penetrating thesecond insulation film PAS can be performed as the same step thusrealizing the reduction of the number of photolithography steps.

Further, among the group of electrodes which constitute the second pixelelectrodes UPX, the electrode which is arranged close to the first drainsignal line DLL is extended in a state that a portion thereof isoverlapped to the first drain signal line DLL thus forming the holdingcapacitance Custl between the second pixel electrode UPX and the firstdrain signal line DLL. Further, the second pixel electrode UPX which isarranged close to the second drain signal line DLR is extended in astate that a portion thereof is overlapped to the second drain signalline DLR thus forming the holding capacitance Custr between the secondpixel electrode UPX and the second drain signal line DLR.

Further, the above-mentioned holding capacitance Custl is formed at aportion substantially equal to a portion where the above-mentionedholding capacitance Cbstl is formed, while the above-mentioned holdingcapacitance Custr is formed at a portion substantially equal to aportion where the above-mentioned holding capacitance Cbstr is formed.This structure is provided for enhancing the capacitance stabilizationeffect.

Then, on the surface of the substrate GLS1 formed in this manner, anorientation film OIL is formed in a state that the orientation film OILcovers at least the whole pixel regions, wherein the orientation filmOIL is brought into direct contact with liquid crystal so as todetermine the initial orientation direction of the molecules of theliquid crystal.

In the liquid crystal display device having such a constitution, thepixel region is constituted of an inner region which is surrounded bythe first drain signal line DLL, the second drain signal line DLR andthe gate signal lines (scanning signal lines) GL.

With respect to operational voltages for display, a signal voltage ofthe above-mentioned first drain signal line DLL is applied to the firstplaner pixel electrode BPX which occupies the substantially whole regionof the pixel area from the first thin film transistor TFTL through thecontacts CNT1 and CNT2 during a period in which a gate ON voltage isapplied to the gate signal line GL, that is, during the selectionperiod. Further, in the same gate ON period as described above, a signalvoltage of the second drain signal line DLR is applied to the secondpixel electrode UPX from the second thin film transistor TFTR throughthe contact CNT2. Then, the liquid crystal is driven in response to thevoltages from the above-mentioned first pixel electrode BPX and thesecond pixel electrode UPX.

Then, the first pixel electrode BPX and the second pixel electrode UPXare overlapped to each other by way of the first insulation film GI andthe second insulation film PAS and the holding capacitance Cp is formedby the overlapped portion. Further, the first pixel electrode BPXextends to portions below the first drain signal line DLL and the seconddrain signal line DLR by way of the first insulation film GI thusforming the holding capacitance Cbstl and the holding capacitance Cbstrrespectively. On the other hand, the second pixel electrode UPX extendsto portions above the first drain signal line DLL and the second drainsignal line DLR by way of the second insulation film PAS thus formingthe holding capacitance Custl and the holding capacitance Custrrespectively.

FIG. 1B is an equivalent circuit diagram which is depicted correspondingto the above-mentioned constitution, wherein Cgsl, Cgsr respectivelyindicate the parasitic capacitance of the first thin film transistorTFTL and the parasitic capacitance of the second thin film transistorTFTR.

FIG. 4A and FIG. 4B show drive waveforms of respective signals suppliedto the pixels having the above-mentioned constitution. Using these drivewaveforms, effects which the above-mentioned holding capacitances Cbstl,Custl, Cbstr, Custr are expected to perform are explained.

FIG. 4A is a view showing drive waveforms of the above-mentioned firstthin film transistor TFTL and FIG. 4B is a view showing drive waveformsof the above-mentioned second thin film transistor TFTR.

Scanning (gate) voltages Vg in FIG. 4A and FIG. 4B are in common and, inrespective drawings, a voltage Vdl of the first drain signal line DLLand a voltage Vdr of the second drain signal line DLR are explicitlyshown. Further, in respective drawings, the source voltage Vbpx of thefirst pixel electrode BPX and the source voltage Vupx of the secondpixel electrode UPX which are outputs of the first thin film transistorTFTL and the second thin film transistor TFTR are also explicitly shown.

The source voltages Vbpx, Vupx generate a voltage drop (αVb) as shown inthe following formula I when the gate voltage Vg is lowered from an ONstate to an OFF state.ΔVb=(Cgsl+Cgsr)/(Cbstl+Custl+Cbstr+Custr+Cgsl+Cgstr)×Vg  (equation 1)

As can be clearly understood from the equation 1, the pixel capacitanceCp is not included in the above-mentioned lowered voltage. This isbecause that when two thin film transistors TFT are formed in one pixel,the pixel capacitance Cp assumes a floating potential and does notaffect the above-mentioned voltage drop. Further, when the holdingcapacitances Cbstl, Custl, Cbstr, Custr are not provided between theabove-mentioned respective drain signal lines DL, the value of thevoltage drop becomes equal to Vg and the source potential becomes equalto or lower than the OFF voltage of the gate thus giving rise to anerroneous operation.

On the other hand, as can be clearly understood from FIG. 4A and FIG.4B, one signal line voltage Vdl and the other signal line voltage Vdrare always set symmetrical with respect to the reference voltage Vc andhence, when four holding capacitance values, that is, Cbstl, Custl,Cbstr, Custr can be set to the substantially equal value, the sourcevoltages Vbps, Vupx are operated in a stable manner with respect to thedrain voltage amplitude.

Such an operation is explained in detail further in conjunction with thecross-sectional view shown in FIG. 2.

First of all, in the cross-sectional view shown in FIG. 2A, when aso-called OFF gate signal is supplied to the gate signal line GL,charges stored in the first pixel electrode BPX and the second pixelelectrode UPX assume a floating state.

That is, the first video signal from the first drain signal line DLL issupplied to the first pixel electrode BPX which occupies approximatelythe whole region of the pixel region through the TFTL and via a contacthole CNT1 and a contact hole CNT2. On the other hand, the second videosignal from the second drain signal line DLR is supplied to the secondpixel electrode UPX from the contact hole CNT2 through the TFTR.

An electric field is generated based on the voltage difference betweenthe first pixel electrode BPX and the second pixel electrode UPX.Accordingly, the transmissivity of the liquid crystal LC is changedcorresponding to the voltage difference. Since the second pixelelectrode UPX is overlapped to the first pixel electrode BPX, with astacked film constituted of the first insulation film GI and the secondinsulation film PAS sandwiched therebetween, the pixel capacitance Cp isformed in the overlapped portion. Since this overlapped portion has nooverlapping relationship with other lines, for example, the gate signalline GL, the first drain signal line DLL and the second drain signalline DLR, when the TFTL and the TFTR are set to an OFF state, the chargestored in this capacitance Cp assumes a floating state.

FIG. 2B shows the constitutions of the holding capacitance elementsCustl, Cbstl, Custr and Cbstr.

First of all, as described previously, the signal voltage of the firstdrain signal line DLL is transmitted to the first pixel electrode BPXand the signal voltage of the second drain signal line DLR istransmitted to the second pixel electrode UPX.

The above-mentioned first pixel electrode BPX extends in a state that aportion of one side thereof is overlapped to the first drain signal lineDLL and a portion of another side thereof is overlapped to the seconddrain signal line DLR by way of the first insulation film GI. Due tosuch a constitution, the first pixel electrode BPX forms the holdingcapacitance Cbstl at a crossing portion of the first pixel electrode BPXand the first drain signal line DLL and forms the holding capacitanceCbstr at a crossing portion of the first pixel electrode BPX and thesecond drain signal line DLR.

Here, the first pixel electrode BPX extends without being overlapped tothe second drain signal line DLR in the pixel region neighboring to theleft side of the first drain signal line DLL and the first pixelelectrode BPX also extends without being overlapped to the first drainsignal line DLL in the pixel region neighboring to the right side of thesecond drain signal line DLR. This is because, when the pixel electrodesBPX are overlapped to the other drain signal lines supplying signals forother pixel regions, the first pixel electrode BPX receives theinfluence from other drain signal lines and hence, the holding potentialis disturbed.

Further, the above-mentioned second pixel electrode UPX extends in astate that a portion of one side thereof is overlapped to the secondpixel electrode UPX and a portion of another side is overlapped to thefirst drain signal line DLL by way of the second insulation film PAS.Accordingly, the holding capacitance Custl is formed at a crossingportion of the second pixel electrode UPX and the first drain signalline DLL and the holding capacitance Custr is formed at the crossingportion of the second pixel electrode UPX and the second drain signalline DLR.

Here, the second pixel electrode UPX extends without being overlapped tothe second drain signal line DLR in the pixel region neighboring to theleft side of the first drain signal line DLL and the second pixelelectrode UPX extends without being overlapped to the first drain signalline DLL in the pixel region neighboring to the right side of the seconddrain signal line DLR. This is because, when the second pixel electrodeUPX is overlapped to other drain signal lines supplying signals forother pixel region, the second pixel electrode UPX receives influencefrom other drain signal lines and hence, the holding potential isdisturbed.

To explain the planar arrangement of the holding capacitances Custl andCbstl which are formed in the vicinity of the above-mentioned firstdrain signal line DLL and the holding capacitances Custr and Cbstr whichare formed in the vicinity of the above-mentioned second drain signalline DLR, as shown in FIG. 1A, the holding capacitances Custl and Cbstland the holding capacitances Custr and Cbstr are arranged alternately toassume a mutually inserted state in a region defined between the drainsignal lines arranged close to each other. Accordingly, the distancebetween the neighboring drain signal lines arranged close to each othercan be decreased and hence, the numerical aperture can be enhanced.Further, the possibility that the short-circuiting is generated betweenthe holding capacitances Custl and Cbstl and between the holdingcapacitances Custr and Cbstr can be easily eliminated.

On the other hand, it is favorable to set the respective values ofholding capacitances Custl, Cbstl, Custr, Cbstr to the substantiallysame value. At least, it is desirable to make the values of the holdingcapacitances Custl, Cbstl, Custr, Cbstr fall altogether within a rangeof 50% to 200%. This can be realized, for example, by setting areas ofcrossing portions where the pixel electrode UPX or BPX and the drainsignal line DLL or DLR cross each other substantially equal with respectto the holding capacitances Custl, Cbstl, Custr, Cbstr. The reason whysuch a constitution is desirable is explained hereinafter.

A large charge stored in the pixel capacitance Cp which is formed of thefirst pixel electrode BPX and the second pixel electrode UPX assumes afloating state when the scanning signal of the gate signal line GL whichdrives the first thin film transistor TFTL and the second thin filmtransistor TFTR is turned off.

Accordingly, for example, when only the holding capacitance Cbstl isformed between the first drain signal line DLL and the first pixelelectrode BPX, during the holding period, the source potential of thefirst pixel electrode BPX is largely changed through the holdingcapacitance Cbstl corresponding to the potential of the first drainsignal line DLL.

In driving the pixels according to this embodiment, amplitude of thevideo signal in the first drain signal line DLL and amplitude of thevideo signal in the second drain signal line DLR exhibit the sameabsolute value while the directions of the amplitudes are set oppositeto each other. Therefore, by forming the holding capacitance Cbstrbetween the second drain signal line DLR and the first pixel electrodeBPX, the change of the potential of the first pixel electrode BPX withrespect to the amplitudes of the respective video signals can besuppressed.

In the same manner, by setting the holding capacitances Custl and Custrto the substantially same value for stabilizing the source potential ofthe second pixel electrode UPX, the operation of the second pixelelectrode UPX can be stabilized. Embodiment 2.

FIG. 5A is a plan view showing another embodiment of the constitution atthe above-mentioned pixel region. Further, FIG. 5B is an equivalentcircuit diagram depicted geometrically corresponding to the constitutionshown in FIG. 1A. Further, FIG. 6A is a cross-sectional view taken alonga line VI (a)-VI (a) in FIG. 5A and FIG. 6B is a cross-sectional viewtaken along a line VI(b)-VI(b) in FIG. 5A.

FIG. 5A is a view corresponding to FIG. 1A of the embodiment 1 and theconstitution of FIG. 5A differs from the constitution of FIG. 1A in thatholding capacitance signal lines STL are formed, and the holdingcapacitance signal line STL is used as another electrode of the holdingcapacitance Cbst which uses the first pixel electrode BPX as oneelectrode thereof and, further, the holding capacitance signal line STLis used as another electrode of the holding capacitance Cust which usesthe second pixel electrode UPX as one electrode thereof.

Here, the holding capacitance signal lines STL are signal lines whichare formed simultaneously at the time of forming the gate signal linesGL. Accordingly, this embodiment adopts the layered structure in whichthe first pixel electrode BPX is formed on a surface of the firstinsulation film GI covering the holding capacitance signal line STL andthe second pixel electrode UPX is formed on a surface of the secondinsulation film covering the first pixel electrode BPX.

Then, the holding capacitance signal line STL is configured such thatthe region to which both of the first pixel electrode BPX and the secondpixel electrode UPX are overlapped and the region to which only thesecond pixel electrode UPX is layered are alternately formed.

Here, the former region forms the holding capacitance Cbst whichstabilizes the potential of the first pixel electrode BPX. On the otherhand, the latter region forms the holding capacitance Cust whichstabilizes the potential of the second pixel electrode UPX.

The constitution of this embodiment is further explained in detail inconjunction with a cross-sectional view shown in FIG. 6.

In FIG. 6A, with respect to the video signal from the first drain signalline DLL, when ON voltage (scanning signal) is applied to the gatesignal line GL, the first thin film transistor TFTL is turned ON and thevideo signal is supplied to the first pixel electrode BPX through thesource electrode SD thereof. On the other hand, the video signal fromthe second drain signal line DLR is, in the same manner, supplied to thesecond pixel electrode UPX through the source electrode SD thereof whenthe second thin film transistor TFTR assumes an ON state.

Since the first thin film transistor TFTL and the second thin filmtransistor TFTR are turned on in response to the same scanning signal(voltage), the video signals from the above-mentioned respective thinfilm transistors are charged to the pixel capacitance Cp which is formedof the second pixel electrode UPX, the first pixel electrode BPX and thesecond insulation film PAS.

Further, in FIG. 6B, the holding capacitance signal line STL is formedon the same layer on which the gate signal line GL is formed, whilebetween the first pixel electrode BPX and the second pixel electrode UPXwhich are formed above the holding capacitance signal line STL, alongthe running direction of the holding capacitance signal line STL, theholding capacitance Cust and the holding capacitance Cbst arealternately formed. This structure is adopted due to the followingreason.

The first pixel electrode BPX is formed on the whole region of thecenter portion of the pixel region except for the slight periphery ofthe pixel region. When the first pixel electrode BPX extends over theholding capacitance signal line STL by way of the first insulation filmGI, only the holding capacitance with respect to the first pixelelectrode BPX is formed and, hence, when the scanning signal is loweredfrom the ON state to the OFF state, an operational point of the secondpixel electrode UPX is lowered due to the influence of the parasiticcapacitance.

Accordingly, in the above-mentioned first pixel electrode BPX which isdisposed above the holding capacitance signal line STL, a plurality ofnotches which are arranged in parallel along the running direction ofthe holding capacitance signal line STL are formed, wherein the holdingcapacitance Cbst is formed at portions of the first pixel electrode BPXwhere notches are not formed and, at the same time, the second pixelelectrode UPX extends in the notched portions and the holdingcapacitance Cust is formed between the extended portions and theabove-mentioned holding capacitance signal line STL.

Here, by setting the capacitance values of the holding capacitance Custand Cbst to the substantially equal value, the most stabilized operationis realized. Since the thickness of the insulation film sandwichedbetween the first pixel electrode BPX and the holding capacitance signalline STL and the thickness of the insulation film sandwiched between thesecond pixel electrode UPX and the holding capacitance signal line STLare different, in setting the above-mentioned capacitance values, it isdesirable to adjust the crossing area of the first pixel electrode BPXand the holding capacitance signal line STL and the crossing area of thesecond pixel electrode UPX and the holding capacitance signal line STLrespectively. With respect to the crossing areas, it is desirable toincrease the area of the second pixel electrode UPX above the holdingcapacitance signal line than to increase the area of the un-notchedportion of the first pixel electrode BPX. More simply, it is desirableto increase a width of the second pixel electrode UPX above the holdingcapacitance signal line than to increase a width of the non-notchedportion of the first pixel electrode BPX.

Accordingly, in this embodiment, the crossing area of the second pixelelectrode UPX and the holding capacitance signal line STL is set largerthan the crossing area of the first pixel electrode BPX and the holdingcapacitance signal line STL.

Embodiment 3

FIG. 7 is a cross-sectional view showing another embodiment of theconstitution at the above-mentioned pixel region and corresponds to FIG.6B.

The constitution shown in FIG. 7 differs from the constitution shown inFIG. 6B in that the second pixel electrodes UPX are connected to eachother along the running direction of the holding capacitance signal lineSTL above the holding capacitance signal line STL. In the same manner asthe embodiment shown in FIG. 6B, in portions except for portions wherethe second pixel electrode UPX is overlapped to the holding capacitancesignal line STL and in the vicinity of the overlapped portion, theabove-mentioned second pixel electrode UPX is constituted of a group ofelectrodes which is constituted of a plurality of electrodes arranged inparallel.

Accordingly, while the portions of the second pixel electrode UPX whichare overlapped with the first pixel electrode BPX are formed above theholding capacitance signal line STL, in the portions of the second pixelelectrode UPX which are not overlapped to the first pixel electrode BPX,the holding capacitance Cust is formed between the second pixelelectrode UPX and the holding capacitance signal line STL.

Here, the holding capacitance Cust is configured such that theoverlapped area of the holding capacitance signal line STL and thesecond pixel electrode UPX can be increased, the capacitance value ofthe holding capacitance Cust can be increased.

Accordingly, in setting the holding capacitance Cust to a given value, aline width of the holding capacitance line STL can be narrowed andhence, the numerical aperture of pixel can be enhanced.

That is, the holding capacitances Cbst and Cust can be alternatelyarranged without gaps along the running direction of the holdingcapacitance signal line STL and hence, the designing necessary forsetting the capacitance values of the respective holding capacitancesCbst and Cust to the same value is facilitated. Further, it is needlessto say that the number of notches formed in the first pixel electrodeBPX is not specifically limited in this embodiment.

The above-mentioned respective embodiments may be used in a single formor in combination. This is because that the advantageous effects of therespective embodiments can be obtained singly or synergistically.

As can be clearly understood from the constitution explained heretofore,according to the liquid crystal display device of the present invention,when the gate potential of the thin film transistor is changed from theON state to the OFF state, it is possible to eliminate the drawbackssuch as the deterioration of operational points which remarkably lowersthe potential of the pixel electrode.

1. A liquid crystal display device comprising: a first electrode towhich a signal is supplied through a first switching element and asecond electrode to which a signal is supplied through a secondswitching element in each pixel region on a substrate, wherein liquidcrystal is driven in response to a potential difference between thefirst electrode and the second electrode, wherein the first electrode isformed as one electrode of a first holding capacitance which isconstituted by sandwiching an insulation film between the firstelectrode and a signal line, and the second electrode is formed as oneelectrode of a second holding capacitance which is constituted bysandwiching an insulation film between the second electrode and a signalline, wherein a signal line which constitutes another electrode of thefirst holding capacitance and a signal line which constitutes anotherelectrode of the second holding capacitance are formed of a capacitancesignal line, and wherein a signal line which constitutes anotherelectrode of the first holding capacitance and a signal line whichconstitutes another electrode of the second holding capacitance areformed of a capacitance signal line.
 2. A liquid crystal display deviceaccording to claim 1, wherein a capacitance value of the first holdingcapacitance is substantially equal to a capacitance value of the secondholding capacitance.
 3. A liquid crystal display device according toclaim 2, wherein the first electrode and the second electrode are formedof a light transmitting conductive film, the first electrode and thesecond electrode are formed as different layers by way of an insulationfilm, one electrode is formed on a most portion of the pixel region, andanother electrode is formed of a group of electrodes which areoverlapped to one electrode.
 4. A liquid crystal display devicecomprising: a first electrode to which a signal is supplied from a firstdrain signal line through a first switching element and a secondelectrode to which a signal is supplied from a second drain signal linethrough a second switching element in each pixel region on a substrate,wherein liquid crystal is driven in response to a potential differencebetween the first electrode and the second electrode, wherein the liquidcrystal display device includes holding capacitance signal lines, theholding capacitance signal lines are overlapped to a plurality portionsof the first electrodes by way of a first insulation film, and theholding capacitance signal lines are overlapped to the second electrodesat regions between a plurality portions of the first electrodes by wayof a first insulation film and a second insulation film.
 5. A liquidcrystal display device according to claim 4, wherein with respect tooverlapped areas where the holding capacitance signal line is overlappedwith the first electrode and the second electrode, the overlapped areawhere the holding capacitance signal line is overlapped with the secondelectrode is larger than the overlapped area where the holdingcapacitance signal line is overlapped with the first electrode.
 6. Aliquid crystal display device according to claim 4, wherein the firstelectrode and the second electrode are formed of a light transmittingconductive film, the first electrode and the second electrode are formedas different layers by way of an insulation film, one electrode isformed on a most region of the pixel region, and another electrode isformed of a group of electrodes which are overlapped to one electrode.